Adherent metal oxide coating forming a high surface area electrode

ABSTRACT

An implantable electrode having a strong, adherent surface coating of iridium oxide or titanium nitride on a platinum surface, where the platinum surface has a surface area of at least five times that of a smooth shiny platinum surface of the same geometry. The iridium oxide coating may be formed on platinum by a physical deposition process, such as sputtering. A gradient coating of iridium oxide ranging in composition from pure platinum to pure iridium oxide is produced by sputtering.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 10/655,772, now U.S. Pat. No. 7,571,011 issued on Aug. 4, 2009 toZhou, et al., which claims the benefit of U.S. Provisional ApplicationSer. No. 60/467,789, filed on May 1, 2003, the disclosures of all ofwhich are incorporated herein by reference.

FEDERALLY SPONSORED RESEARCH

This invention was made with government support under grant No.R24EY12893-01, awarded by the National Institutes of Health. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

This invention relates to metal oxide forming an adherent high surfacearea coating on an electrode surface.

BACKGROUND OF THE INVENTION

It has been known for 200 years that muscle contraction can becontrolled by applying an electrical stimulus to the associate nerves.Practical long-term application of this knowledge was not possible untilthe recent development of totally implantable miniature electroniccircuits that avoid the risk of infection at the sites of percutaneousconnecting wires. One example of this modern technology is the cardiacpacemaker.

A metal of choice in electrode manufacturing has traditionally beentitanium. On a fresh titanium surface, however, oxygen ions react withthe titanium anode to form an oxide layer. Once a finite oxide thicknesshas been formed on the surface, polarization increases. The oxide filmdeveloped on the surface of a titanium electrode is irreversible. Itcannot be reduced to the original metal by passing a charge in thereverse direction. Hence, pure titanium metal is a poor choice forelectrodes since it forms a semi-conductive oxide on its surface beforeand during electrical stimulation. Platinum and stainless steel undergoirreversible dissolution during stimulation as well.

Titanium oxidation reactions are several times more likely in anoxidative environment than those of platinum or platinum alloys, but athousand times less so than those of stainless steel. Due to the expenseof platinum metal and the requirement for large amounts of metal inpatch-type electrodes, costs may be too high for the routine use ofplatinum electrodes.

The charge storage capacity, C, is calculated according to the equationC=(k)(ε)(A/d), where k is the dielectric constant of the film, ε is thepermissivity in vacuum, A is the true surface area of the film, and d isthe thickness of the porous material, it can be seen that in order toachieve a large charge storage capacity (C), the porosity of thedielectric may be maximized with a large film surface area. Numeroustypes of cardiac pacing and defibrillation electrodes have beendeveloped with these factors in mind, utilizing various configurationsand materials asserted to promote lower stimulation thresholds and toimprove electrical efficiencies. Thus, for implantable electrodeapplications, it is desirable to minimize the electrical impedance atthe electrode-tissue interface by increasing the intrinsic surface areaof the electrode or by reducing formation of the capsule of inactivetissue that surrounds and isolates the electrode from living tissue.Schaldach discusses in detail the selection criteria for implantableelectrodes. See M. Schaldach, “Fractal Coated Leads: Advanced SurfaceTechnology for Genuine Sensing and Pacing,” Progress in BiomedicalResearch, 259-272, June 2000.

Microporous electrodes based on sintered titanium, sintered titaniumnitride, and microporous carbon or graphite have been used with somedegree of success. However, the electrode reactions in aqueous solutionsinvolve significant gas generation similar to the behavior of titanium.Abrading or sandblasting electrode surfaces is a broadly used method toachieve surface area enhancement. For example, French Patent No.2,235,666 relates to a stainless steel electrode tip that is sanded toincrease surface area and reduce the impedance of the electrode.

Other methods have also been used. U.S. Pat. No. 5,318,572 relates to a90% platinum-10% iridium porous electrode with recess slots in the shapeof a cross and at least one, preferably two variably-sized, porouscoatings of 20 to 80 micron diameter 90% platinum-10% iridium spheresdeposited on the surface of the electrode. On top of this structure, areactively sputtered coating of titanium nitride was applied. U.S. Pat.No. 4,156,429 describes a means for increasing the reactive surface areaby forming a highly porous sintered electrode body consisting of abundle of fibers, preferably of platinum but alternatively of ELGILOY,titanium, or a platinum-iridium alloy. Conversely, the fibers may beencompassed within a metallic mesh to yield 70% to 97% porosity. U.S.Pat. No. 5,203,348 relates to defibrillation electrodes that can beformed on titanium ribbons or wires with a sputtered outer layer ofplatinum, or a silver core in a stainless steel tube with a platinumlayer formed onto the tube. U.S. Pat. No. 5,230,337 discloses that thecoating is preferably made by sputtering to increase the surface area ofthe electrode.

U.S. Pat. No. 5,178,957 relates to electrodes and a method of makingelectrodes including pretreatment of the surface by sputter-etching andsputter-depositing a noble metal on the surface. U.S. Pat. No. 5,074,313relates to a porous electrode with an enhanced reactive surface whereinsurface irregularities are introduced to increase surface area by glowdischarge or vapor deposition upon sintered wires. U.S. Pat. No.4,542,752 describes a platinum or titanium substrate coated with aporous sintered titanium alloy that in turn is coated with a porouscarbon. The latter was claimed to promote tissue ingrowth and providelow polarization impedance. U.S. Pat. No. 4,784,161 relates to making aporous pacemaker electrode tip using a porous substrate, where theporous substrate is preferably a non-conductive material such as aceramic or a polymer made porous by laser drilling, sintering, foaming,etc. to result in pores 5 to 300 microns in depth. U.S. Pat. No.4,603,704 features a hemispherical electrode made of platinum ortitanium, coated with a porous layer consisting of a carbide, nitride,or a carbonitride of at least one of the following metals: titanium,hafnium, molybdenum, niobium, vanadium, or tungsten. U.S. Pat. No.4,281,668 discloses a vitreous carbon or pyrolytic carbon electrode thatis superficially activated, e.g., by oxidation, for microporosity. Theelectrode is then coated with a biocompatible ion-conducting,hydrophobic plastic.

Despite the numerous means of increasing the surface area to reducepolarization losses and after potentials and the use of noble metals andtheir alloys as electrodes as described above, with varying degrees ofsuccess, there remain significant problems pertaining to polarizationlosses and sensing difficulties. In order to make further improvementsto the electrode, stable oxides of some of these noble metals have beenemployed as a coating.

It is known that certain metals, metallic oxides, and alloys are stableduring electrolysis, and that these metals are useful in a variety ofelectrode applications, such as chlor-alkali electrolysis (see U.S. Pat.No. 5,298,280). Such metals typically include the elements of theplatinum group; namely, ruthenium, rhodium, palladium, osmium, iridium,and platinum. These metals are not suitable for construction of theentire electrode, since their cost is prohibitive. Therefore, thesemetals or their alloys, or as metallic oxides, have been applied as athin layer over a base member made of Ti, Ta, Nb, Hf, Zr, or W. Thesemetals are much less expensive than platinum group metals and they haveproperties that render them corrosion resistant. However, as previouslymentioned, they lack good surface electroconductivity because of theirtendency to form a surface oxide having poor electroconductivity.

U.S. Pat. No. 5,683,443 discloses implantable stimulation electrodes forliving tissue stimulation where the titanium electrodes have metaloxides, such as iridium oxide, applied as coatings on an electrodesurface, where the surface area has been increased by mechanicalshaping, abrasion by sandblasting, or roughening by chemical etching.The patent also discloses surface area enhancement by applying coatingsof metal oxides by virtue of the preferred fit which is possible usingmixed sized metal oxide molecules in a lattice arrangement. Thus, asingle metal oxide produces a mono-lattice with gaps, but a mixed metaloxide with differently sized molecules produces a binary lattice wherethe gaps of the mono-lattice may be filled by the smaller of the twomolecules.

Iridium oxide may be used as a protective coating for metallicelectrodes made of platinum, platinum iridium alloy, stainless steel,stainless steel alloys, titanium, titanium alloys, tantalum, or tantalumalloys. U.S. Pat. No. 4,677,989 discloses a metallic electrode that ismade of a metal other than iridium that is coated with iridium oxide toreduce corrosion and to increase charge capacity while being thin, thusallowing charge to flow to living tissue from the electrode. Formationof an iridium oxide coating by a solution chemistry deposition processis discussed.

U.S. Pat. No. 5,632,770 discloses an implantable device with a poroussurface coating having an active surface that is substantially largerthan the geometric shape of the electrode. The enhanced surface area wasachieved by a three dimensional fractal-like geometry that increased thesurface area by 1000 fold. A coating such as iridium nitride or iridiumoxide applied by vacuum technology, particularly vapor deposition, suchas reactive cathode sputtering, CVD, PVD, MOCVD, or ion plating isdisclosed.

Electroplated or sputtered iridium oxide on a metal surface cracks anddelaminates after a short period of electric current pulsing.Thermo-prepared iridium oxide has no such problem. Some commerciallyavailable pacemaker leads lasted for eight years of implantation. It isbelieved that thermo-prepared iridium oxide has better adhesion to thesubstrate (usually Ti). High temperature will fuse the iridium oxidecoating to the substrate. However, since this process requires hightemperature, it is not compatible with materials that are hightemperature sensitive.

EIC Laboratories of Norwood, Mass. electroplated iridium oxide on amachined surface of a solid disk of platinum and electroplated iridiumoxide on a highly polished thin film of platinum; both had low adhesionstrength. Some electrodes had delamination of iridium oxide with staticexposure at room temperature to 10% saline. Some electrodes developedcracks after a few voltage cycles (i.e., cyclic voltammetry) −0.6V to+0.8V. The plated layer appears dense and smooth. Sputtered iridium oriridium oxide exhibits the same limitations.

U.S. patent application Ser. No. 10/226,976, titled “Platinum Electrodeand Method for Manufacturing the Same,” to Zhou, now U.S Pat. No.6,974,533, discloses an alternative fractal platinum material andmethods of manufacture and is incorporated herein by reference in itsentirety. Due to the superior electrical characteristics of platinum aswell as its biocompatibility and stability, platinum is a preferredmaterial for electrodes in harsh environments, such as in a human body.However, because electrodeposited platinum, also called “brightplatinum”, has a smooth surface when deposited at a slow depositionrate, its surface area is limited by the geometry of the electrode, andtherefore, it is not efficient for transferring electrical charge.

Another form of platinum, known as “platinum black,” is widely known. Itis deposited at a high rate and demonstrates high porosity, lowstrength, a rough surface that has lower bulk density, and lessreflectivity of visible light than bright platinum. U.S. Pat. No.4,240,878 describes a method of plating platinum black.

Platinum black may require additives, such as lead, which promote rapidplating. Lead, however, is a neurotoxin and cannot be used in biologicalsystems. Because platinum black is weak, the thickness of theelectroplated layer is quite limited. Thick layers of platinum black areinherently weak and readily flake.

Another form of platinum is “platinum gray,” which possessesintermediate properties between those of bright platinum and platinumblack. Formed by electrodeposition at an intermediate rate between thatutilized for bright platinum and platinum black, it possesses thedesirable high surface area that is characteristic of its fractalmorphology. It is strong and can be deposited in thick layers onimplantable electrodes. However, it suffers from long-term degradationwhen exposed to living tissue and when subjected to higher chargedensity stimulation.

It is desired to have the benefit of a high surface area electrode thatis comprised of porous platinum and that is coated with an inert, strongcoating that vigorously adheres to the substrate and therefore does notflake off during long-term exposure to living tissue.

SUMMARY OF THE INVENTION

This invention is directed to an implantable, coated electrode havinghigh surface area for greater ability to transfer charge and also havingsufficient physical and structural strength to withstand physicalstresses encountered during use.

This and other aspects of the present invention which may become obviousto those skilled in the art through the following description of theinvention are achieved by a coated electrode and method formanufacturing the electrode wherein the electrode has a fractal surfacecoating of platinum, known as “platinum gray,” with a increase insurface area of at least 5 times when compared to shiny platinum of thesame geometry and also having improved resistance to mechanical stresswhen compared to platinum black. The platinum gray fractal surface iscoated with either a gradient or discrete coating of an inert material,such as iridium oxide.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a scanning electron micrograph of iridium oxidedeposited on a platinum gray surface.

FIG. 2 presents a scanning electron micrograph of iridium oxidedeposited on a rough platinum surface.

FIG. 3 is a three-electrode electroplating cell with a magnetic stirrer.

FIG. 4 is an electroplating system with constant voltage control orscanned voltage control.

FIG. 5 is cyclic voltammograms of electroplated iridium oxide.

FIG. 6 is a plot of impedance as a function of frequency for selectplatinum samples.

FIG. 7 illustrates the formation of a gradient coating in cross-section.

FIG. 8 is a schematic representation of a cross-section through a coatedporous electrode.

FIG. 9 illustrates a two-target sputtering apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, an illustrative example of an iridium oxide coatingon a platinum gray surface of an electrode, according to the presentinvention, is shown having a fractal surface with a surface areaincrease of between 5 and 500 times the surface area for a shinyplatinum surface of the same size. An implantable electrode and methodfor manufacturing the electrode wherein the electrode has a strongadherent surface coating of iridium oxide or titanium nitride on aplatinum surface, which demonstrates an increase in surface area of atleast five times when compared to smooth platinum of the same geometry.The iridium oxide coating may be formed on platinum by a physicaldeposition process. such as sputtering. The process of electroplatingthe iridium oxide surface coating is accomplished by voltage controlprocesses. A gradient coating of iridium oxide ranging in compositionfrom essentially pure platinum to essentially pure iridium oxide isproduced by sputtering. An implantable electrode comprising a roughenedconductive substrate: and a rough surface coating on the substrate. Theimplantable electrode, wherein said surface coating has a surface areaof between 5 and 500 times the corresponding surface area resulting fromthe basic geometric shape.

The implantable electrode, wherein the surface coating is comprised ofiridium oxide. The implantable electrode has a charge storage capacitythat is greater than 35 mC/cm². The implantable electrode, where thesurface coating is comprised of titanium nitride.

The implantable electrode, wherein the titanium nitride is a sputteredcoating. The implantable electrode. wherein the surface coating iscomprised of a graded coating. The implantable electrode, wherein thegraded coating is comprised of platinum gray and iridium oxide.

The implantable electrode, wherein the electrode is suitable for avisual prosthesis. The implantable electrode, wherein the visualprosthesis is a retinal electrode array.

The implantable electrode, wherein the roughened conductive substrate iscomprised of platinum gray.

The implantable electrode, wherein the surface coating is biocompatible.

The implantable electrode, wherein the surface coating has a thicknessof at least 1 micron. The implantable electrode, wherein the surfacecoating has a thickness of less than 1 micron.

The implantable electrode, wherein at least a portion of the surfacecoating has fractal morphology. The implantable electrode, wherein atleast a portion of said roughened conductive substrate is comprised of afractal morphology.

The implantable electrode, wherein the surface coating is a sputteredcoating. The implantable electrode, wherein the roughened conductivesubstrate is comprised of a fractal morphology. The implantableelectrode. wherein the surface coating is an electroplated coating.

The implantable electrode. wherein the surface coating is comprised ofelectroplated iridium oxide and the conductive substrate is comprised ofplatinum gray. FIG.2 presents a scanning electron photomicrograph of arough platinum surface that is coated with iridium oxide. It is clearthat the electrode surface may, in an alternative embodiment, be a roughsurface that is achieved on a surface such as platinum, which has beenroughened by abrasion, such as by abrasion blasting or chemical etching.FIGS. 1 and 2 are scanning electron micrographs taken on a JEOL JSM5910scanning electron microscope. Iridium oxide coats the fractal surface ofthe platinum gray with a cauliflower-like morphology with feature sizesranging from 0.5 to 15 microns. Each branch of such structure is furthercovered by smaller and smaller features of similar shape. The featuresparticles on the surface layer may be in the nanometer range. This roughand porous fractal structure increases the electrochemically activesurface area of the platinum surface when compared to an electrode witha smooth platinum surface having the same geometric shape.

Because no impurities or other additives, such as lead, which is aneurotoxin and cannot be used in an implantable electrode, need to beintroduced during the plating process to produce platinum gray, thesurface is essentially pure platinum.

A quasi-gradient coating 30 is achieved by plating iridium oxide 28 onthe extremely porous surface of platinum gray 26, as illustrated in FIG.7. The average coating composition increases from pure platinum to pureiridium oxide as one moves from the bottom surface 32 to the top surface34 of the sample. The resulting iridium oxide coating is very adherentto the platinum gray base material (not illustrated).

Platinum gray can also be distinguished from platinum black based on theadhesive and strength properties of the thin film coating of thematerials. Adhesion properties of thin film coatings of platinum grayand platinum black on 500 microns diameter electrodes were measured on aMicro-Scratch Tester (CSEM Instruments, Switzerland). A controlledmicro-scratch was generated by drawing a spherical diamond tip of radius10 microns across the coating surface under a progressive load from 1 to100 millinewtons with a 400 micron scratch length. At a critical loadthe coating stared to fail. It was found that platinum gray had acritical load of over 60 millinewtons while platinum black had acritical load of less than 35 millinewtons.

The platinum gray layer is introduced as an adhesion layer between metalsubstrate and iridium oxide. This mid-layer is micro-rough andmicro-porous which provides at least two benefits: (a) larger surfacearea due to its fractal structure to accommodate more iridium oxide massin a less dense form; and (b) providing a rough surface for iridiumoxide deposition and promote good adhesion to substrate. The iridiumoxide coating on such modified surface is less dense and has betteradhesion. This iridium oxide layer provides very high charge storagecapacity for pulse stimulation. The electrode can also be used aselectrochemical sensors. This mid-layer is introduced by electroplatingplatinum or other metals such as iridium, rhodium, palladium, gold,tantalum, titanium, niobium or their alloys.

Other surface modification techniques (such as chemical, electrochemicaland physical etching, low temperature thin-film deposition methods,etc.) can also be used to provide a rough surface for iridium oxideplating.

Referring to FIGS. 3 and 4, a method to produce adherent iridium oxideaccording to the present invention is described, comprising anelectroplating cell 12, generally, connecting a common electrode 2,which is preferably comprised of platinum, and a working electrode 4 tobe plated The working electrode 4 (which is the cathode during platinumgray plating and is the anode during iridium oxide plating) is connectedto a potentiostat 6, which is the power source, with a computer monitor8, for control and process monitoring of either the current or voltageof the potentiostat 6. The common electrode 2, working electrode 4,reference electrode 10 for use as a reference in controlling thepotentiostat 6 and electroplating solution 13 are placed in anelectroplating cell 12 having a means for mixing or agitating 14 theelectroplating solution 13. Electrical power is supplied to theelectrodes with a constant voltage or a scanned voltage to drive theelectroplating process. Referring to FIG. 3, the electroplating cell 12,is preferably a 30 ml to 150 ml four-neck glass flask or beaker, thecommon electrode 2, is preferably a large surface area platinum wire orplatinum sheet, the reference electrode 10 is preferably a Ag/AgClelectrode (silver, silver chloride electrode), the working electrode 4that is to be plated, can be any suitable material depending on theapplication and can be readily chosen by one skilled in the art.Preferable examples of the working electrode 4 to be plated include butare not limited to platinum, iridium, rhodium, gold, tantalum, titanium,niobium or their alloys. The plated iridium oxide is dark blue in color.

The means for mixing or agitating 14 is preferably a magnetic stirrer,as shown in FIG. 3. The plating solution 13 is preferably 3 to 6 mM(millimole) iridium chloride in potassium carbonate solution buffered toa pH of 10-11. The preferable plating temperature is approximately 24°C. to 26° C.

An electroplating system with constant voltage and scanned voltagecontrol is shown in FIG. 4. While, constant current, pulsed voltage orpulsed current can be used to control the electroplating process (suchas a pulsed, rectangular potential waveform between 0.0V and 0.55V vs.saturated calomel electrode (SCE), with a 0.2 to 5 second dwell at eachlimit), constant voltage control of the plating process has been foundto be most preferable. The most preferable voltage range to produceadherent iridium oxide has been found to be +0.45V to +0.65V. Applyingvoltage in this range with the above solution 13 yields a plating rateof about 2 to 4 mC/cm²/min, which is the preferred range for the platingrate of iridium oxide. Constant voltage control also allows an array ofelectrodes to be plated in parallel simultaneously, achieving the samesurface layer thickness for each electrode.

A constant voltage is applied on the common electrode 2 as compared tothe reference electrode 10 preferably using an EG&G PAR M273 modelpotentiostat 6. The response current of the common electrode 2 isrecorded by a the computer monitor 8. After a specified time, preferably15-90 minutes, and most preferably 30 minutes, the voltage is turned offand the working electrode 4 is thoroughly rinsed in deionized water.

The electrochemical impedance of the electrode array with the surfacecoating of platinum gray is measured in a saline solution. Thecharge/charge s density and average plating current/current density arecalculated by integrating the area under the plating current as afunction of time curve. Scanning Electron Microscope (SEM) EnergyDispersive X-ray Analysis is performed on selected electrodes. SEMmicrographs of the plated surface are taken showing the fractal surfacemorphology. Energy Dispersive X-ray Analysis demonstrates that thesample consists of platinum and iridium oxide.

Cyclic voltammetry and Electrochemical Impedance Spectroscopy in PBS(0.126 M NaCl, 0.1 M NaH₂PO₄/Na₂HPO₄ at pH=7.2) saturated with argon ornitrogen measures the charge storage capacity of the iridium oxideelectrodes. FIG. 5 presents the cyclic voltammograms of an iridium oxideplated microelectrode array. The electrodes have four different sizeswith 500, 350, 250 and 150 μm diameter thin-film platinum disks coatedwith iridium oxide on platinum gray. Two pairs of current peaks withinthe potential range of −0.6V to +0.8V, are presented on thevoltammograms, FIG. 5, are typically observed for iridium oxidereduction and oxidation reactions. The charge storage capacity,calculated by integrating the area under the voltammograms, has anaverage value of 102 mC/cm² (115, 103, 100 and 91 mC/cm² for the fourdifferent electrode sizes, i.e., 500, 350, 250, and 150, respectively).The scan rate is 50 mV/sec in 0.1 M phosphate buffered saline at a pH of7.4. This charge storage capacity is much higher that the 25 mC/cm²achieved for either iridium oxide on a smooth surface or for activatediridium oxide. Higher charge storage capacity, for example, exceeding 25mC/cm², on smooth electrodes results in cracks and delamination of theiridium oxide layer. The average rate of deposition for theelectroplated iridium oxide is 3.4 mC/cm²/min for constant voltageplating compared at 1.2 mC/cm²/min pulsed voltage plating using 0.5second pulses. The average rate of deposition for the electroplatediridium oxide is 3.4 mC/cm²/min for constant voltage plating compared at1.2 mC/cm²/min pulsed voltage plating using 0.5 second pulses.

FIG. 6 presents a comparison of the impedance spectra for threedifferent surfaces: smooth platinum 44 before plating, after roughplatinum 46 plating for 30 minutes, and after iridium oxide 48 plated onrough platinum, each having the same geometrical surface area of20×10^(−4 cm) ² for a polyimide array, where the plating wasaccomplished at +0.5V versus Ag/AgCl. The electrode impedance decreasedafter rough platinum plating and was further reduced after iridium oxideplating on the rough platinum surface. The charge storage capacitymeasured in the electrode's capacitance, which is proportional to theelectrode surface area, was determined to increase more than 200 timesfor the iridium oxide plated surface, as compared with unplatedelectrodes of the same diameter. In an alternative embodiment, titaniumnitride is coated on the rough platinum surface, preferably bysputtering deposition.

The iridium oxide plating solution is prepared as presented below.

Electrolyte Concentrations

Molecular Weight (g- Quantity Chemical mol) Concentration (milliMoles)IrCl₄ 334 1.42 g/l 4.25 H₂O₂ aqueous solution 34 10 ml/l 88 (30 wt %)(COOH)₂ 2H₂O 126 5 g/l 40 K₂CO₃ to pH 10.5 99 ~34 g/l 340

Solution Preparation

1. Dissolve 0.15 g iridium chloride hydrate in 100 ml deionized water,stirring for 30 minutes.

2. Add 1 ml aqueous hydrogen peroxide solution (30 weight percent),stirring for 10 minutes.

3. Add 0.5 g oxalic acid (COOH)₂ 2H₂O, stirring for 10 minutes.

4. Add anhydrous potassium carbonate to adjust the solution pH to 10.5.

5. Maintain the final solution at 100 to 25° C. for at least 2 days tostabilize.

Hydrogen peroxide enables oxide film deposition at lower currentdensity. The complex of iridium salt with oxalic acid is believed toprovide a stable solution for electrochemical oxidation of iridium toiridium oxide during electroplating and solution storage for about 3months without precipitation of iridium oxide. The iridium oxide platingproceeds by the following reaction:[Ir(COO)₂(OH)₄]⁻²→IrO₂+2CO₂+2H₂O+2e⁻

These procedures are presented in the following references, K. Yamanaka,“Anodically Electrodeposited Iridium Oxide Films (AEIROF) from AlkalineSolutions for Electrochromic Display Devices,” Jpn. J. Appl. Phys. 28632-37 (1989); R. D. Meyer, T. H. Nguyen, R. D. Rauh, and S. F. Cogan,“Electrodeposition of Iridium Oxide Charge Injection Electrodes,”Proceeding of BMES/EMBS'99, Atlanta, Oct. 13-16, 1999; S. Marzouk, etal., “Electrodeposited Iridium Oxide pH Electrode for Measurement ofExtracellular Myocardial Acidosis during Acute Ischemia,” Anal. Chem.,70 5054-61 (1998).

EXAMPLE 1

An array with 16 platinum disks embedded in silicone rubber aselectrodes was plated with iridium oxide. Each platinum disk had adiameter of 500 μm and had an exposed metal surface of 20×10⁻⁴ cm². Allthe electrodes in the array were shorted to a common contact point forplating. The platinum disk electrodes were pulsed at 0.35 mC/cm², 1 msbiphasic square wave pulse at 50 Hz for 6 hours in saline. Thisstimulation removed surface contamination and polymer residue. The arraywas cleaned and electrochemically conditioned in 0.5 M H₂SO₄ solution by10 cycles of voltage scans from −0.2V to +1.4V vs AgCl with 100 mV/secscan rate. A thin layer of rough platinum (about 5 μm) was electroplatedfor 30 minutes on the platinum disk surface. Next, 10 cyclicvoltammetric scans were carried out on the platinum surface in 4.25 mMiridium chloride solution to confirm the reduction and oxidation ofiridium oxide. Then, iridium oxide was electroplated in the samesolution at a constant voltage of +0.5 V as a function of AgCl for 25minutes on the platinum surface. The resulting current density wasdetermined to be 0.36 mA/cm². The iridium solution is purged with argonor nitrogen gas for 20-30 minutes prior to plating. A magnetic stirrerwas used to agitate the solution during plating. The solutiontemperature was maintained at room temperature (i.e., about 22-24° C.).The resulting electrodes had very high capacitance, with an average of15 mF/cm². The platinum disks had an average capacitance of 0.05 mF/cm²,while the plated rough platinum surfaces had capacitance of 0.7 mF/cm².

EXAMPLE 2

A thin-film platinum polyimide array was used for iridium oxide plating.The array contained 16 electrodes of four different sizes, 500, 350, 250and 150 μm diameter thin-film platinum disks, as exposed electrodesurfaces. All of the electrodes in the array were shorted to commoncontact points for the plating. The platinum disk electrodes were firstelectrochemically cleaned by bubbling oxygen over the surface while thesurface was held at +3V vs AgCl in 0.5 M H₂SO₄ for 5 sec. The surfacewas then cleaned by bubbling with hydrogen at −2.5V vs AgCl in 0.5 MH₂SO₄ for 5 seconds. This removed surface contamination and polymerresidue. The array was electrochemically conditioned in 0.5 M H₂SO₄solution by 10 cycles of voltage scans from −0.2V to +1.4V vs AgCl with100 mV/sec scan rate. A thin layer of rough platinum (about 10 μm thick)was electroplated for 20 minutes on the platinum disk surface. Next, 10cyclic voltammetric scans were carried out on the platinum surface in4.25 mM iridium oxide solution to confirm the reduction and oxidation ofiridium oxide peaks. Next, iridium oxide was electroplated on theplatinum surface in the same solution at a constant voltage of +0.5V vsAgCl for 15 minutes. The resulting current density was measured as 0.27mA/cm². The iridium solution was purged with argon or nitrogen gas for20-30 minutes prior to plating. A magnetic stirrer agitated the solutionduring plating. The solution was maintained at room temperature (about22° to 24° C.). The resulting electrode had very high capacitance withan average of 5.1 mF/cm². The thin-film platinum disks had an averagecapacitance of 0.023 mF/cm², while the rough platinum surfaces had anaverage capacitance of 0.96 mF/cm².

EXAMPLE 3

A thin-film platinum polyimide array was used for iridium oxide plating.The array contained 16 electrodes of 200 μm diameter thin film platinumdisks as the exposed electrode surface. All the electrodes in the arraywere shorted to a common contact point for plating. The platinum diskelectrodes were first electrochemically cleaned by bubbling oxygen onthe surface at +3V vs AgCl in 0.5 M H₂SO₄ for 5 seconds. Then they werecleaned by bubbling hydrogen on the surface at −2.5V vs AgCl in 0.5 MH₂SO₄ for 5 seconds. This removed surface contamination and polymerresidue. The array was electrochemically conditioned in 0.5 M H₂SO₄solution by 10 cycles of voltage scans from −0.2V to +1.4V vs AgCl witha 100 mV/sec scan rate. A thin layer of platinum gray (about 10 μmthick) was electroplated for 20 minutes on the platinum disk surface.Next, an iridium layer was electrodeposited on the platinum graysurface. Finally, iridium was electrochemically activated to formiridium oxide by cyclic voltammetric scans within the potential windowof −0.6V to +0.8V vs AgCl in saline. The solution was purged with argonor nitrogen gas for 20 to 30 minutes prior to iridium oxide activation.A magnetic stirrer agitated the solution during plating. The solutionwas maintained at 22° to 24° C.

In an alternative embodiment, iridium oxide is introduced during theplatinum plating process such that the iridium oxide layer is depositedon the high surface area platinum gray to yield a gradient compositioncoating that varies approximately linearly from essentially pureplatinum gray to essentially pure iridium oxide. It is clear that thegradient coating may vary in composition step-wise or in other than anon-linear manner. It is also clear that the composition need not bepure on either surface and that the gradient coating may equally well becomprised of a fraction of platinum and a complementary fraction ofiridium oxide.

Illustrated schematically in FIG. 8 is a cross section of a high surfacearea coated electrode, generally 50. The rough platinum electrode 54 iscoated with a high surface area coating 52. The enhancement of thesurface area of the rough platinum surface, which is preferably platinumgray, but which may be a chemically or mechanically roughened surface,is achieved by coating with the high surface area coating 52, which ispreferably iridium oxide, but which in an alternative embodiment may besputtered biocompatible material having a high surface area, such astitanium nitride.

A further alternative embodiment utilizes two-target sputtering forgradient coating deposition, generally illustrated in FIG. 9. Theplatinum and the iridium oxide are deposited by sequential gradedco-sputtering. Sputtering is a relatively low-temperature method forfabricating an electrode 24 with two materials deposited in a gradientmanner. This method creates an electrode 24 or electrode array that hasplatinum metal adjacent to the base of the electrode substrate, andiridium oxide at the surface of the electrode 24. The resultingstructure is an approximately linear gradient from essentially pureplatinum to essentially pure iridium oxide, although it is clear thatthe gradient coating may be started and/or terminated at any point inthe gradient that is less than the essentially pure material. It is alsoto be understood that the gradient composition may vary in compositionin a manner other than linearly. In alternative embodiments, the base ofthe electrode may be comprised of platinum gray, another metal, polymer,or ceramic. For neural stimulation, the preferred embodiment is a thinfilm flex circuit base made from a biocompatible material, such aspolyimide, preferably with platinum or alternately with titaniumconductive traces.

The electrode material gradient is sputtered such that the essentiallypure platinum is deposited for about 300 to 3000 angstroms, although itis envisioned that the platinum layer may be microns thick in analternative embodiment. The iridium oxide is phased in untilapproximately 50% platinum and 50% iridium is being deposited. Then theplatinum flux is reduced until only iridium oxide is deposited,resulting in an electrode surface of 100% iridium oxide.

To achieve this sequential graded co-sputtering, the electrodes arepreferably placed on a rotating carousel 18 in a sputtering machine thatis capable of very rapid rotation and can sputter from two targetssimultaneously. One target is an RF target for the sputtering of theiridium oxide. 20 An RF target is necessary as opposed to a DC target,since iridium oxide is a poor conductor. Since platinum is a goodelectrical conductor, the other target is preferably DC, although inalternative embodiments it may be RF. Rapid carousel rotation on theorder of 100 rpm minimizes the layering effect of the sputteringdeposition process. Each electrode passes the platinum sputtering target16 and the iridium oxide sputtering target 20, alternately, in rapidsuccession, thereby forming a series of extremely thin alternatinglayers. Reducing the amount of deposited material to 10 to 40 angstromsper rotation yields a smoother gradient.

Deposition is preferably done with an argon background pressure of 1 to10 mTorr. Initially only the platinum sputtering target 16 is powered.In the preferred embodiment, a very high power and high rate ofdeposition is used such that the platinum is deposited in a very roughfilm on the electrode 24, maximizing the surface area. However, afterthis rough base is achieved, power to the platinum sputtering target 16is reduced to minimize the layering effect. Alternatively, a layer ofplatinum grey is plated on the electrode 24 prior to sputtering toprovide a rough, high surface area base.

Then the iridium oxide target 20 is powered at a low level to beginintroducing iridium oxide into the platinum deposition.

In yet another embodiment, the deposition rate of both target materialsis modulated with shutters rather than by power modulation. Achievingdeposition rate modulation requires an independent shutter for eachtarget. The shutter 21 over the iridium oxide target is slowly openedwhile the shutter over the platinum target 17 is slowly closed resultingin a graded transition from platinum to iridium oxide. Near the end ofthe deposition process, the iridium oxide target shutter 21 iscompletely open and the platinum target shutter 17 is completely closed,resulting in pure iridium oxide at the electrode surface 24.

Since the simultaneous wet etching of platinum and iridium oxide is verydifficult, the final steps in electrode fabrication preferably utilizethe lift-off process for patterning the deposited material gradient.

An electrode 24 or electrode array having a gradient coating by thisprocess has the excellent charge delivery properties possessed byiridium oxide while maintaining the robustness afforded by theunderlying platinum layer.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A dual coated high surface area implantable metalelectrode comprising: a smooth metal substrate, suitable forimplantation in an eye, having a first surface area and having ageometric shape; an electrically conductive coating comprised of fractalplatinum and not platinum black or shiny platinum, deposited on saidsmooth metal substrate, suitable for implantation in an eye, having asecond surface area, said electrically conductive coating having saidsecond surface area greater than five times said first surface area ofsaid smooth metal substrate; and a further protective fractal coatingdeposited on said electrically conductive coating, suitable forimplantation in an eye.
 2. The implantable electrode of claim 1, whereinsaid implantable metal electrode has a charge storage capacity that isgreater than 35 mC/cm².
 3. The implantable electrode of claim 1, whereinsaid further protective fractal coating is comprised of titaniumnitride.
 4. The implantable electrode of claim 3, wherein said titaniumnitride is a sputtered coating.
 5. The implantable electrode of claim 1,wherein said implantable metal electrode is suitable for a visualprosthesis.
 6. The implantable electrode of claim 5, wherein said visualprosthesis is a retinal electrode array.
 7. The implantable electrode ofclaim 1, wherein said surface coating is biocompatible.
 8. Theimplantable electrode of claim 1, wherein said further protectivefractal coating has a third surface area of between 5 and 500 times saidthe corresponding first surface area of said smooth metal substrate. 9.The implantable electrode of claim 1, wherein said further protectivefractal coating has a thickness of at least 1 micrometer.
 10. Theimplantable electrode of claim 1, wherein said further protectivefractal coating is comprised of iridium oxide.